Electrooptic light modulator device

ABSTRACT

This invention provides an electrooptic light modulator device with a nonlinear optical component consisting of a transparent solid medium of a naphthoquinodimethane compound such as 11,11-di(n-hexyldecylamino)-12,12-dicyano-2,6-naphthoquinodimethane: ##STR1##

This patent application is a divisional of patent application Ser. No.864,203, filed May 19, 1986, now U.S. Pat. No. 4,707,305 issued Nov. 17,1987; which is a continuation-in-part of patent application Ser. No.748,583, filed June 25, 1985, now U.S. Pat. No. 4,640,800 issued Feb. 3,1987.

BACKGROUND OF THE INVENTION

In addition, the properties of organic and polymeric materials can bevaried to optimize other desirable properties, such as mechanical andthermoxidative stability and high laser damage threshold, withpreservation of the electronic interactions responsible for nonlinearoptical effects.

Thin films of organic or polymeric materials with large second-ordernonlinearities in combination with silicon-based electronic circuitryhave potential as systems for laser modulation and deflection,information control in optical circuitry, and the like.

Other novel processes occurring through third-order nonlinearity such asdegenerative four-wave mixing, whereby real-time processing of opticalfields occurs, have potential utility in such diverse fields as opticalcommunications and integrated circuit fabrication.

Of particular importance for conjugated organic systems is the fact thatthe origin of the nonlinear effects is the polarization of theπ-electron cloud as opposed to displacement or rearrangement of nuclearcoordinates found in inorganic materials.

Nonlinear optical properties of organic and polymeric materials was thesubject of a symposium sponsored by the ACS division of polymerChemistry at the 18th meeting of the American Chemical Society,September 1982. Papers presented at the meeting are published in ACSSymposium Serial 233, American Chemical Society, Washington, D.C. 1983.

The above-recited pulbications are incorporated herein by reference.

Of more specific interest with respect to the present inventionembodiments is prior art relating to tetracyanoquinodimethane compounds,such as U.S. Pat. Nos. 3,115,506; 3,226,389; 3,408,367; 3,681,353;3,687,987; 3,953,874; 4,229,364; and 4,478,751.

There is continuing research effort to develop new nonlinear opticalorganic systems for prospective novel phenomena and devices adapted forlaser frequency conversion, information control in optical circuitry,light valves and optical switches. The potential utility of organicmaterials with large second-order and third-order nonlinearities forvery high frequency application contrasts with the bandwidth limitationsof conventional inorganic electrooptic materials.

Accordingly, it is an object of this invention to provide organiccompositions which are characterized by a large delocalized conjugatedπ-electron system which can exhibit nonlinear optical response.

It is another object of this invention to provide a novel class oforganic compounds which is characterized by a charge asymmetricquinodimethane conjugated structure.

It is a further object of this invention to provide high performancenonlinear optical substrates.

Other objects and advantages of the present invention shall becomeapparent from the accompanying description and examples.

DESCRIPTION OF THE INVENTION

One or more objects of the present invention are accomplished by theprovision of a novel class of charge asymmetric conjugatednaphthoquinodimethane compositions.

The term "charge asymmetric" as employed herein refers to the dipolaritycharacteristic of organic molecules containing an electron-withdrawinggroup which is in conjugation with an electron-donating group.

Illustrative of the invention class of compositions are quinodimethanecomopounds corresponding to the structural formula: ##STR2## where R andR¹ are substituents selected from hydrogen and aliphatic, alicyclic andaromatic groups containing between about 1-20 carbon atoms, and at leastone of the R substituents is an electron-donating group, and at leastone of the R¹ substituents is an electron-withdrawing group.

Illustrative of R¹ electron-withdrawing substituents are cyano, nitro,trifluoromethyl, tricyanoethylene, and the like. R¹ alternatively can behydrogen or an aliphatic, cycloaliphatic or aromatic group such asmethyl, chloroethyl, methoxyethyl, pentyl, decyl, 2-propenyl,2-propynyl, cyclohexyl, phenyl, tolyl, and the like.

The ═CR¹ R¹ ] moiety can also represent a cyclic structure which iselectron-withdrawing, such as: ##STR3##

Illustrative of R electron-donating substituents are amino, alkylamino,alkenylamino, alkynylamino, alkoxy, thioalkyl, phosphinyl, and the like.R alternatively can be hydrogen or an aliphatic, cycloaliphatic oraromatic group as described for the R¹ substituent above.

The [RRC ═ moiety can also represent a cyclic structure which iselectron-donating, such as: ##STR4##

Illustrative of preferred naphthoquinodimethane compounds are those inwhich the pair of R substituents are the same electron-donating groups,and the pair of R¹ substituents are the same electron-withdrawinggroups.

An important aspect of the present invention is the provision of anaphthoquinodimethane type compound which has utility as a chargeasymmetric component of nonlinear optical media.

Naphthoquinodimethane structures of preference are those correspondingto the formula: ##STR5## where R is hydrogen or an alkyl group.Illustrative of alkyl groups are methyl, ethyl, propyl, isopropyl,butyl, isobutyl, pentyl, decyl, hexadecyl, eicosyl, and the like. Alkylgroups containing between about 1-20 carbon atoms are preferred. The NR₂group can also represent a heterocyclic structure such as piperidyl,piperazyl or morpholinyl.

The ═C(NR₂)₂ moiety in the formulae can constitute a heterocyclicradical in which the two amino groups taken together with the connectingmethylidene carbon atom form a cyclic structure such as imidazoline inthe naphthoquinodimethane compound: ##STR6##

The naphthoquinodimethane compounds can also contain substituents whichhave one or more optically active asymmetric centers, such as chiralisomeric structures corresponding to the formula: ##STR7##

In all of the naphthoquinodimethane structural formulae illustratedherein the cyclic groups can have one or more of the hydrogen positionson the ring carbon atoms replaced with a substituent such as alkyl,halo, alkoxy, phenyl, and the like, or can be integrated as part of amore complex fused polycyclic ring structure.

A compound such as11,11-diamino-12,12-dicyano-naphtho-2,6-quinodimethane can besynthesized from 2,6-dimethylnaphthalene in accordance with thefollowing series of chemical reaction steps: ##STR8##

The synthesis of 11,11,12,12-tetracyanonaphtho-2,6-quinodimethane from2,6-dimethylonaphthalene or 2,6-bis(bromomethyl)naphthalene is describedin J. Org. Chem., 28, 2719(1963) and J. Org. Chem., 39 (No. 8),1165(1974).

NONLINEAR OPTICAL PROPERTIES

A quinodimethane compound of the present invention can be utilized as acharge asymmetric component of a nonlinear optical medium.

Thus, in another embodiment this invention can provide a nonlinearoptical organic medium exhibiting a X.sup.(2) susceptibility of at leastabout 1×10⁻⁶ esu, and wherein the substrate comprises anoncentrosymmetric configuration of molecules having anaphthoquinodimethane structure corresponding to the formula: ##STR9##where R and R¹ are substituents selected from hydrogen and aliphatic,alicyclic and aromatic groups containing between about 1-20 carbonatoms, and at least one of the R substituents is an electron-donatinggroup, and at least one of the R¹ substituents is anelectron-withdrawing group. R and R¹ are substituents as previouslydefined and illustrated.

In another embodiment this invention can provide a nonlinear opticalorganic medium exhibiting a X.sup.(2) susceptibility of at least about1×10⁻⁶ esu, an absence of interfering fluorescence in the wavelengthrange between about 0.3-3 μm, an optical loss less than about 0.1decibel per kilometer, a response time less than about 10⁻¹³ second,phase matching of fundamental and second harmonic frequencies, adielectric constant less than about 5, and wherein the medium comprisesa noncentrosymmetric configuration of molecules having a quinodimethaneconjugated structure corresponding to the formula: ##STR10## where R isa substituent selected from hydrogen and alkyl groups.

The naphthoquinodimethane molecules can have an external field-induceduniaxial molecular orientation in a host liquid medium, or an externalfield induced stable uniaxial molecular orientation in a host solidmedium. A substrate of unaligned quinodimethane molecules exhibits thirdorder nonlinear optical response.

In another embodiment this invention can provide an opticallytransparent medium comprising a noncentrosymmetric or centrosymmetricconfiguration of an11,11-diamino-12,12-dicyanonaphtho-2,6-quinodimethane type or an11,11-di(alkylamino)-12,12-dicyanonaphtho-2,6-quinodimethane type ofmolecules.

In a further embodiment this invention can provide a nonlinear opticalmedium comprising a solid polymeric matrix having incorporated therein adistribution of 11,11-diamino-12,12-dicyanonaphtho-2,6-quinodimethane or11,11-di(alkylamino)-12,12-dicyanonaphtho-2,6-quinodimethane molecules.

The term "Miller's delta" as employed herein with respect to secondharmonic generation (SHG) is defined by Garito et al in Chapter 1,"Molecular Optics:Nonlinear Optical Properties Of Organic And PolymericCrystals"; ACS Symposium Series 233 (1983).

The quantity "delta" (δ) is defined by the equation: ##EQU1## whereterms such as X_(ii).sup.(1) are the linear susceptibility components,and d_(ijk), the second harmonic coefficient, is defined through##EQU2##

The Miller's delta (10⁻² m⁻² /c at 1.06 μm) of various nonlinear opticalcrystalline substrates are illustrated by KDP (3.5), LiNbO₃ (7.5), GaAs(1.8) and 2-methyl-4-nitroaniline (160).

Such comparative figures of merit are defined over the frequency rangeextending to zero frequency, or equivalently DC, and the polarizationelectrooptic coefficient as described in the publication by Garito et alrecited above.

The term "fluorescence" as employed herein refers to an optical effectin which a molecule is excited by short wavelength light and emits lightradiation at a longer wavelength. The fluorescence effect is describedwith respect to liquid dye lasers in Optoelectronics, An Introduction,pages 233-236, Prentice Hall International, Englewood Cliffs, N.J.(1983).

The term "optical loss" as employed herein is defined by the equation:

    αL=10 log (I.sub.o /I)

where

α=attenuation coefficient ratio of lost light per unit length

L=sample length

I_(o) =intensity of incident light

I=intensity of transmitted light.

The term "optical scattering loss" is defined and measuredquantitatively by ##EQU3## where T₁ is the transmission of opticalradiation through the test sample between polarizers perpendicular toeach other, and T₁₁ is the transmission between polarizers parallel toeach other.

The term "response time" as employed herein refers to numerous physicalmechanisms for nonlinear optical responses and properties of nonlinearoptical materials. The fastest intrinsic response time to lightradiation is a physical mechanism based on electronic excitationscharacterized by a response time of about 10⁻¹⁴ -10⁻¹⁵ seconds. Responsetime is a term descriptive of the time associated with optical radiationcausing promotion of an electron from the electronic ground state to anelectronic excited state and subsequent de-excitation to the electronicground state upon removal of the optical radiation.

The term "phase matching" as employed herein refers to an effect in anonlinear optical medium in which a harmonic wave is propagated with thesame effective refractive index as the incident fundamental light wave.Efficient second harmonic generation requires a nonlinear optical mediumto possess propagation directions in which optical medium birefringencecancels the natural dispersion, i.e., the optical transmission offundamental and second harmonic frequencies is phase matched in themedium. The phase matching can provide a high conversion percentage ofthe incident light to the second harmonic wave.

For the general case of parametric wave mixing, the phase matchingcondition is expressed by the relationship:

    n.sub.1 ω.sub.1 +n.sub.2 ω.sub.2 =n.sub.3 ω.sub.3

where n₁ and n₂ are the indexes of refraction for the incidentfundamental radiation, n₃ is the index of refraction for the createdradiation, ω₁ and ω₂ are the frequencies of the incident fundamentalradiation and ω₃ is the frequency of the created radiation. Moreparticularly, for second harmonic generation, wherein ω₁ and ω₂ are thesame frequency ω, and ω₃ is the created second harmonic frequency 2ω,the phase matching condition is expressed by the relationship:

    n.sub.ω =n.sub.2ω

where nω and n₂ω are indexes of refraction for the incident fundamentaland created second harmonic light waves, respectively. More detailedtheoretical aspects are described in "Quantum Electronics" by A. Yariv,chapters 16-17 (Wiley and Sons, New York, 1975).

The term "dielectric constant" as employed herein is defined in terms ofcapacitance by the equation: ##EQU4## where C=capacitance when filledwith a material of dielectric constant

C_(o) =capacitance of the same electrical condenser filled with air

The term "external field" as employed herein refers to an electric ormagnetic field or mechanical stress which is applied to a medium ofmobile organic molecules, to induce dipolar alignment of the moleculesparallel to the field or stress direction.

The term "optically transparent" as employed herein refers to an opticalmedium which is transparent or light transmitting with respect toincident fundamental light frequencies and created light frequencies. Ina nonlinear optical device, a present invention nonlinear optical mediumis transparent to both the incident and exit light frequencies.

The fundamental concepts of nonlinear optics and their relationship tochemical structures can be expressed in terms of dipolar approximationwith respect to the polarization induced in an atom or molecule by anexternal field, as summarized in the ACS Symposium Series 233 (1983).

FIELD-INDUCED MICROSCOPIC NONLINEARITY

The microscopic response, or electronic susceptibility β, and itsfrequency dependence or dispersion, is experimentally determined byelectric field induced second harmonic generation (DCSHG) measurementsof liquid solutions or gases as described in "Dispersion Of TheNonlinear Second Order Optical Susceptibility Of Organic Systems",Physical Review B, 28 (No. 12), 6766 (1983) by Garito et al, and theMolecular Crystals and Liquid Crystals publication cited above.

In the measurements, the created frequency ω₃ is the second harmonicfrequency designated by 2ω, and the fundamental frequencies ω₁ and ω₂are the same frequency designated by ω. An applied DC field removes thenatural center of inversion symmetry of the solution, and the secondharmonic signal is measured using the wedge Maker fringe method. Themeasured polarization at the second harmonic frequency 2ω yields theeffective second harmonic susceptibility of the liquid solution and thusthe microscopic susceptibility β for the molecule.

The present invention class of novel organic compounds exhibitsextremely large values of β because of a noncentrosymmetricnaphthoquinodimethane structure. Illustrative of this class of compoundsare 11,11-di(hexydecylamino)-12,12-dicyano-2,6-napththo-quinodemethaneand 11,11-di(dimethylamino)-12,12-dicyano-2,6-naphthoquinodimethane:##STR11##

The theory and practice of high performance nonlinear optical media,with specific reference to quinodimethane compounds, is elaborated incopending patent application Ser. No. 748,617, filed June 25, 1985; nowU.S. Pat. No. 4,707,303, issued 11-17-87, incorporated herein byreference.

LANGMUIR-BLODGETT DEPOSITION TECHNIQUE

The Langmuir-Blodgett technique is reviewed in J. Macromol. Sci.-Rev.Macromol. Chem., C21(1), 61(1981); incorporated herein by reference.

An extensive elaboration of Langmuir-Blodgett technology is published inThin Solid Films, Vol. 99(1983), which includes papers presented at theFirst International Conference On Langmuir-Blodgett Films, Durham, GreatBritain, Sept. 20-22, 1982; Elsevier Sequoia S. A., Lausanne;incorporated herein by reference.

In 1917 Irving Langmuir developed the experimental and theoreticalconcepts which underlie our understanding of the behavior of organicmolecules in insoluble monolayers on the surface of water. Langmuirdemonstrated that long-chain fatty acids on the surface of water formfilms in which the molecules occupy the same cross-sectional areawhatever the chain length of the molecules. The films are one moleculethick, and the molecules are oriented at the water surface, with thepolar functional group immersed in the water and the long nonpolar chaindirected nearly vertically up from the water surface.

This understanding of the nature of insoluble monolayers was facilitatedby the development of a surface balance, which is associated withLangmuir's name.

In 1919 Langmuir reported a development in which fatty acid monolayerson water surfaces were transferred to solid supports such as glassslides. In 1933 Katherine Blodgett announced the discovery thatsequential monolayer transfer could be accomplished to form built-upmultilayer films, i.e., unitary laminate structures now universallyreferred to as "Langmuir-Blodgett films".

Grunfeld et al in Thin Solid Films, 99, 249 (1983) demonstrate theapplication of a Langmuir-Blodgett layer as a potentially usefulintegrated optics component by employing the optical absorptionanisotropy of a diacetylene film in a polarization mode filter.

The present invention naphthoquinodimethane compositions are amenable toLangmuir-Blodgett deposition procedures for the formation of monolayerand multilayer continuous film coatings on solid substrates such asoptical glass. The coated substrates exhibit nonlinear opticalproperties.

The following examples are further illustrative of the presentinvention. The components and specific ingredients are presented asbeing typical, and various modifications can be derived in view of theforegoing disclosure within the scope of the invention.

Fluorescence activity in a nonlinear optical substrate is measured byPerkin-Elmer Fluorescence Spectroscopy Model No. MPF-66 or LS-5.

Optical loss exhibited by a nonlinear optical substrate is measured byoptical time domain reflectometry or optical frequency-domainreflectometry as described in "Single-mode Fiber Optics" by Luc B.Jeunhomme, Marcel Dekker Inc., N.Y., 1984. It is also measured by themethod described in "The Optical Industry And Systems PurchasingDirectory", Photonics, 1984. The scattering optical loss isquantitatively measured by the ratio of perpendicular transmission toparallel transmission of a He--Ne laser beam through the nonlinearsample which is placed between crossed polarizers.

The response time of a nonlinear optical substrate is calculated by themethod described in "Optoelectronics; An Introduction" by P. J. Dau,Editor, Prentice-Hall International.

The dielectric constant of a nonlinear optical substrate is measured bythe methods described in Chapter XXXVIII of "Technique of OrganicChemistry", Volume I, Part III, (Physical Methods of Organic Chemistry)by Arnold Weissberger, Editor, Interscience Publishers Ltd., New York,1960.

EXAMPLE 1

This Example illustrates the preparation of11,11-diamino-12,12-dicyano-2,6-naphthoquinodimethane in accordance withthe present invention.

Ten grams of 11,11,12,12-tetracyano-2,6-naphthoquinodimethane preparedby a synthetic scheme as previously described [J. Org. Chem., 39 (No.8), 1165 (1974)] and 2 liters of tetrahydrofuran are placed in athree-necked three-liter flask equipped with a mechanical stirrer, anitrogen inlet, a drying tube and a gas-inlet connected to an anhydrousammonia gas tank. Ammonia gas is bubbled through the stirred solutionfor three days at room temperature. The crude product in precipitateform is filtered from the reaction mixture, washed with distilled water,and recrystallized from N,N-dimethylformamide (DMF)-water to yield highpurity 11,11-diamino-12,12-dicyano-2,6-naphthoquinodimethane product. DCinduced second harmonic generation can achieve a second order nonlinearoptical susceptibility β of about 300×10⁻³⁰ esu, an opticalsusceptibilty X.sup.(2) of about 10×10⁻⁶ esu, and a Miller's delta ofabout 4 square meters/coulomb.

When the NLO substrate is centrosymmetric in macroscopic configuration,it can exhibit a nonlinear optical susceptibility X.sup.(3) of about1×10⁻¹⁰ esu, a response time of less than 10⁻¹³ second, an absence offluorescence in the wavelength range between about 0.3-3 μm, an opticalloss less than about 0.1 decibel per kilometer, and a dielectricconstant less than about 5.

EXAMPLE 11

This Example illustrates the preparation of11,11-di(n-butylamino)-12,12-dicyano-2,6-naphthoquinodimethane inaccordance with the present invention.

A three-necked three-liter flask equipped with a mechanical stirrer, anitrogen inlet, a drying tube, and an addition funnel is charged with 10grams (0.03 moles) of 11,11,12,12-tetracyano-2,6-naphthoquinodimethaneand two liters of tetrahydrofuran. Twenty-nine grams (0.12 moles) ofn-butylamine in 100 ml of tetrahydrofuran is added dropwise into theflask, and the resulting mixture is stirred for three days at roomtemperature. The resulting THF solution is concentrated on a rotaryevaporator.

The crude product in precipitate form is separated by filtration, washedwith distilled water, neutralized with 10% solution of ammoniumhydroxide, washed with water, and then recrystallized from DMF-water toyield 11,11-di(n-butylamino-12,12-dicyanonaphthoquinodimethane.

This compound is aligned in a melt-phase in a DC field by applying about15 Kvolts/cm, and cooled slowly to freeze the aligned molecularstructure in the DC field. The aligned molecular structure is opticallytransparent and can exhibit a nonlinear optical susceptibiilty β ofabout 350×10⁻³⁰ esu, a X.sup.(2) of about 1.5×10⁻⁶ esu, and a Miller'sdelta of about 4 square meters/coulomb.

In a transparent solid medium in which the molecules are randomlydistributed, the product can exhibit a nonlinear optical susceptibilityX.sup.(3) of about 1×10⁻¹⁰ esu. The other properties are similar tothose described for the Example I product.

EXAMPLE III

This Example illustrates the preparation of11,11-di(n-hexyldecylamino)-12,12-dicyano-2,6-naphthoquinodimethane.

Following the procedure of Example II,11,11-di(n-hexadecylamino)-12,12-dicyano-2,6-naphthoquindodimethane isprepared by employing a tetrahydrofuran solution containing ten grams of11,11,12,12-tetracyano-2,6-naphthoquinodimethane and thirty-two grams ofn-butylamine. The second order nonlinear susceptibility β is about200×10⁻³⁰ esu after alignment of molecules in a DC field, or afteralignment of molecules by the Langmuir-Blodgett Technique in which amonolayer or severals layers of molecules are deposited on a glasssubstrate.

EXAMPLE IV

This Example illustrate the use of11,11-di(n-hexyldecylamino)-12,12-dicyano-2,6-naphthoquinodimethane as aguest molecule in a polymer substrate.

Ten grams of11,11-di(n-hexadecylamino)-12,12-dicyano-2,6-naphthoquinodimethane and90 grams of poly(methyl methacrylate) are dissolved in 400 ml ofmethylene chloride. A film (2 mil) is cast from this solution on a glassplate coated with indium tin oxide. Another glass plate coated withindium tin oxide is placed on the film, and then the film is heated toabout 150° C. A DC field is applied to align the molecules, and the filmis cooled slowly in the applied field to yield an aligned polymer alloywhich can have a second order nonlinear susceptibility β of about300×10⁻³⁰ esu.

What is claimed is:
 1. An electrooptic light modulator device with anonlinear optical component comprising a transparent solid medium of anaphthoquinone composition corresponding to the formula: ##STR12## whereR is a substituent selected from hydrogen and alkyl groups containingbetween about 1-20 carbon atoms.